1. Field of the Invention
The present invention relates to rust preventing treatment of metal-plated steel material, such as zinc-plated, and zinc-alloy-plated steel materials.
2. Description of Prior Arts
Conventionally, chromate treatments have been widely used for rust prevention of metal materials, and various treating baths and methods have been developed for the chromate treatment. Basically these baths and methods are common and similar to each other in that they use chromic acid, dichromate or their salts.
Thus in the conventional chromate treating bath, there are present hexavalent chromium ions which react with the metal of the surface being treated to form the so-called chromate protective film and a film of hydrated compound of chromium oxide, entrapping the hexavalent chromium therein, which gradually dissolves in water to repair damaged surface portions. This is called "sealing action". Among various treatments, the chromate treatment has been most widely practised because it is very simple to perform and produces satisfactory corrosion resistance.
However, the chromate treatment has a problem of environment pollution in connection with the disposal of the used solution containing the hexavalent chromium which is effective component of the treating solution, and has defects that deterioration of the formed film is relatively rapid and adhesion with paint coating is not satisfactory.
Therefore, in recent years, strong demands have been made for developments of new surface treatments replacing the chromate treatment and overcoming the problems and defects confronted with by the chromate treatment.
The present inventors conducted various extensive studies for development of a new type of surface treatment of metal products, particularly metal-plated steel products, such as zinc-plated and zinc-alloy-plated, and steel products.
As a result, the present inventors developed a new surface treatment method using an aqueous solution of tannic acid as disclosed in Japanese Patent Publication No. Sho 51-2902.
According to this new treating method, not only paint-adhesion but also corrosion resistance of the metal-plated steel products can be improved by simple immersion in the treating solution just as in the chromate treating method. Further, the above new method has an advantage that disposal of the used solution is very easy because it does not contain heavy metals.
The present inventors have made further studies on rust preventing effect of the tannic acid on the metal, and found that particularly in case of zinc and its alloys, the tannic acid in an aqueous solution reacts with the metal very quickly and forms a film of aggregation polymer which hardly dissolves in water. This aggregation polymer film dissolves slightly and gradually in water in a similar manner to the gradual dissolution of the chromate film, and repairs the damaged surface portions, namely "sealing action", to produce disirable rust preventing effect.
However, when the aggregation polymer is exposed to water or an aqueous solution for a long time, the tannic acid component dissolves out of the film, and thus the film becomes no more effective to prevent rust. Further, the film is hard and brittle and, when formed in a thick film, it is very susceptible to cracking and peels off easily.
The present inventors have conducted various studies and experiments to overcome the above defects confronted with by the tannic acid treatment, and have found that the properties of the film can be remarkably improved by adding to the treating solution one or more of inorganic or organic, water-soluble or water-dispersible polymers which are stable when mixed with the tannic acid in the solution.